1. Technical Field
This invention relates to ion sources, and more particularly to controlled kinetic energy ion sources coupled to miniature ion traps or ion trap arrays, and spectroscopy systems based thereon.
2. Description of the Related Art
Time-of-flight (TOF) mass spectrometry is an analytical technique that is widely used because of its simplicity and wide mass range. In an idealized TOF system, ions are initially confined to a small spatial region and are nearly at rest near an electrode. However, in real TOF-based systems, the ions are initially neither nearly at rest nor in a well defined spatial region.
At certain discrete times, generally denoted as t=0, the ions are accelerated by an applied electric field imposed between an acceleration grid and an electrode sheet where the ions initially reside. The ions are then allowed to drift in a zero field region located between the acceleration grid and a detector until they reach the detector. The arrival time of the ions can be related to their mass because the heavier ions achieve a lower velocity while in the acceleration zone as compared to lighter ions. Thus, the method requires that the ions be pulsed in time or in a beam that is chopped at high frequency. There are many configurations of time-of-flight mass spectrometers. For example, some use reflection of the ions in an attempt to compensate for different initial velocities at the start of the acceleration that would otherwise significantly reduce the mass resolution.
The mass resolution of a TOF mass spectrometer depends on the ability to measure the drift time of ions with high precision. One way to achieve this precision is to ensure that all ions have low initial velocities and are spatially localized in a small region at the initial time. An ion trap can be used to achieve this initial condition by trapping and cooling sample ions until the initial time, at which time all ions are released together. Cooling the ions lowers the velocity of the ions. An additional advantage is that ions can be accumulated in the trap between extraction pulses so that the number of ions detected at a given time will be higher, thus increasing sensitivity.
Ion mobility spectrometry (IMS) is another form of chemical analysis that is similar to TOF mass spectrometry, but identifies chemical species based on drift time through a drift channel. The mechanical arrangement for IMS is about the same as in TOF. Ions start at t=0 in a confined region, then are allowed to drift through a constant field region to a detector, with an arrival time inversely proportional to the ion mobility. As with TOF, measurement resolution is improved by spatially localizing the ions in a small region at the initial time.
IMS is performed at higher pressure, even atmospheric pressure, versus a high vacuum for TOF-mass spectrometry. The gas that is present in IMS causes a viscous drag on the ions so it is necessary to have an electric field in the drift region. In practice, the drift and acceleration regions are generally merged into one drift channel. The ions move through the drift region with a velocity that is proportional to the electric field. The proportionality constant is characteristic of the ion but not quite as informative as the mass. Also, the resolution is degraded because of the diffusion that takes place during the drift.
In addition, in IMS the ion velocity is proportional to the applied field, whereas in TOF-mass spectrometry the ion acceleration is proportional to the applied field. IMS has a wide variety of applications currently because it does not require a vacuum system and is the method generally used in airports to test baggage for explosives and drugs, and also by the military for CW detection.
U.S. Pat. No. 6,469,298 to Ramsey et al., entitled “MICROSCALE ION TRAP MASS SPECTROMETER” discloses miniature ion traps including submillimeter traps having improved spectral resolution over earlier small ion traps for mass spectrometry chemical analysis. U.S. patent application Ser. No. 10/801,913 to Whitten et al. entitled “ION TRAP ARRAY-BASED SYSTEMS AND METHODS FOR CHEMICAL ANALYSIS” discloses related ion-trap arrays and is published as 6,933,498 on Aug. 23, 2005. The ion traps disclosed in these references can each have an effective radius r0 and an effective length 2z0, wherein at least one of r0 and z0 are less than 1.0 mm, and a ratio z0/r0 is greater than 0.83. Both r0 and z0 can be less than 1.0 mm. Miniature ion traps allow for the creation of field portable mass spectrometers. However, such devices currently have limited application because translationally hot ions provided by conventional ion sources, such as electrospray or laser ablation and matrix-assisted laser desorption ionization (MALDI), are far too energetic to be trapped in the trap(s). As a result, ions in such systems can only be created with the trap. Consequently, such instruments are generally limited to only electron impact of small volatile organic molecules and gas phase testing.